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That sounds to me like it was probably submitted twice, one got upvoted and then flagged.

As I understand it, you can't downvote posts. So a post should not drop from 48 points to 20.

You can flag posts. That counts as a downvote (or more).

If I had to guess, I'd say the submission got heavily penalized for being trollbait. Either by the mods, or by the community.

Flags affect ranking (which might be what you're getting at), but they don't change the number of votes displayed.

Might be, but I've only seen my submission + I'm 100% sure I saw this precise post in the frontpage with 40+ upvotes :/

There is a search function at the bottom of the page. I will suggest you double check.

Thanks! But yup, searched it and I only see my own submission :/

I wouldn't worry about it too much. I imagine after the many facebook posts these past few days, many people downvote or flag them immediately.